1. Field of the Invention
The present invention relates to a heating element at least including, a heat-resistant base member, a conductive layer having a heater pattern formed on the heat-resistant base member, and a protection layer with an insulating property formed on the conductive layer.
2. Description of the Related Art
A heater in which a line or foil of metal having a high melting point such as molybdenum or tungsten is wrapped around or bonded to a heat-resistant base member made of sintered ceramic such as alumina, aluminum nitride or zirconia has been used for heating semiconductor wafers in manufacturing steps of semiconductor devices.
However, such a heater has drawbacks of being prone to deform or vaporize because the heating element is made of metal, being short-life, and being complicated to assemble (see, the pyrolytic graphite/pyrolytic boron nitride heater from Union Carbide Services provided in “Vacuum” No. 12, (33), p. 53). Furthermore, use of sintered ceramic for the heat-resistant base member causes a problem that the binder in the sintered ceramic becomes impurities.
Then, to prevent such deformation or scattering of impurities due to a heat cycle, a ceramic heater is developed. The ceramic heater has a heat-resistant base member of pyrolytic boron nitride (PBN) having high mechanical strength and enabling high-efficiency heating, and a conductive layer of pyrolytic graphite on the heat-resistant base member (for example, see the pyrolytic graphite/pyrolytic boron nitride heater from Union Carbide Services provided in “Vacuum” No. 12, (33), p. 53, U.S. Pat. No. 5,343,022; Japanese Patent Laid-open (Kokai) No. 05-129210; and Japanese Patent Laid-open (Kokai) No. 06-61335).
An example of a heating element of such a heater is shown in FIG. 4. A heating element 20 has at least a heating portion 20a in which a heater pattern 3a is formed on a plate-shaped heat-resistant base member 21, and a power-supply-terminal portion 20c in which power-supply terminals 3c are formed at the rim of the surface of the heat-resistant base member 21 on which the heater pattern is formed. A protection layer 4 with an insulating property is formed on the heater pattern 3a. To the power-supply terminal 3c, a power-supply member or a power terminal 5 is connected.
However, pyrolytic graphite used for the heating body is prone to undergo corrosion due to oxidation. Pyrolytic graphite has also reactivity with high-temperature gases used in the heating process. For example, hydrogen gas changes pyrolytic graphite into methane gas. Therefore, there is a problem that remaining oxygen or high-temperature gases in the process environment corrodes pyrolytic graphite in the power-supply-terminal portion exposed for power supply, and the power-supply-terminal portion is short life.
To solve the problem, an attempt to locate the power-supply-terminal portion away from the heating portion is made. For example, the following solution is suggested: a power-supply terminal is connected to a power-terminal member via a power-supply member having a heater pattern which produced heat by turning on electricity. Insulating ceramic such as PBN is used for a protection layer covering the heater pattern, thereby preventing overheating of the power-supply-terminal portion to increase longevity of the power-supply terminal (see, Japanese Patent Laid-open (Kokai) No. 11-354260).
Furthermore, the following method is suggested: assembling the power-supply-terminal portion made of carbon with an assembly part and forming a protection layer (see, U.S. Pat. No. 5,343,022; International Publication WO2004/068541).
However, such a heating element has protrusions on the heating surface. It is necessary to provide a space between the heating surface and an object to be heated, which hampers compact design of the heating element. In addition, a protection layer in the vicinity of a connected part assembled from plural components is apt to produce cracks through usage. A conductive layer begins to corrode from the cracks, which causes a problem to shorten the life of the heating element.
Furthermore, when the heating element is used in an environment corroding boride such as using a halide etching gas, there is a drawback that an outermost layer of boride lacks resistance to corrosion, and corrosion of the outermost layer shortens the life of the heating element.
Moreover, an advanced ceramic heater with electrostatic chuck on a heater for holding a semiconductor wafer serving as the object to be heated thereonto has been suggested currently (see, Japanese Patent Laid-open (Kokai) No. 05-129210; Japanese Patent Laid-open (Kokai) No. 06-61335; Japanese Patent Laid-open (Kokai) No. 05-109876). However, occasionally, chuck capability is not exerted sufficiently according to resistivity of the protection layer or the chucked wafer is damaged or cracked. Moreover, heat resistance and corrosion resistance of the heater is also insufficient in the same manner as described above.